QRE: [N8VEM: 2420] Re: Can not get the databuffer chip to work

["Andrew Lynch" <lyn...@yahoo.com>]

Hi!  Thanks!  Yes, the ECB is for IO external to the SBC only.  The SBC will
allow memory writes over the ECB but will not allow memory reads.  There are
a couple of technical limitations to using memory on the ECB, first the ECB
bus interface logic prohibits reads from the ECB if /MREQ is low.  Also,
there is no way to enable a "hole" in the SBC memory map to allow memory on
the ECB to be mapped into the CPU's memory address range.

There are a couple of reasons for this design; first using the ECB for an IO
only bus is incredibly simple and reliable due to an extra wait state in IO
requests compared to memory requests.  Instruction fetches across the ECB is
the worst case scenario and the Z80 has fairly tight timing requirements.
By keeping all the memory requests local to the SBC the design is far more
robust than an SBC ECB system which allows memory devices to be read from
and written to.

Also IO local to the SBC are restricted from being on the ECB as well.  That
means any IO in the range $60-7F are strictly reserved for the SBC's
exclusive use with no way for there to be a conflict with any devices on the
ECB.  This is another simplifying assumption to keep the system robust and
prevent conflicts.

If there is ever a next generation SBC, I will probably relax these
restrictions to open up the design for some advanced builders.  However, the
system has worked fairly well so far and the practical limitations have been
minimal.  Memory mapped peripherals are rare except for video cards and even
those can be implemented as IO devices through the use of latches, etc.

I hope this helps!  Bernie, tell us about your Z80 tester / front panel
device!  That is great and something I think all the builders would like to
see!  Being able to boot strap a ROMless N8VEM from a front panel would be
very cool indeed and would be an incredibly neat addition to the ECB bus
monitor.

Thanks and have a nice day!

Andrew Lynch

> -----Original Message-----
> From: n8...@googlegroups.com [mailto:n8...@googlegroups.com] On Behalf Of
> B
> Sent: Friday, February 06, 2009 3:56 PM
> To: n8...@googlegroups.com
> Subject: [N8VEM: 2420] Re: Can not get the databuffer chip to work
> 
> 
> I think you will find the external data bus is turned off for the address
> range located on the N8vem board. I had the same problem with my ste
> interface and had to strap out the 245 used for the data bus.
> 
> -----Original Message-----
> From: n8...@googlegroups.com [mailto:n8...@googlegroups.com] On Behalf Of
> BZ80
> Sent: 06 February 2009 20:43
> To: N8VEM
> Subject: [N8VEM: 2419] Re: Can not get the databuffer chip to work
> 
> 
> Sorry! Somehow pushed the wrong button and the unfinished post got
> published!! Let me try it again.
> 
> Hi all!
> I have a hardware problem, and I hope someone nows what is going on.
> I have build a periphial board to be able to use external ROMs. I want to
> burn my own programs but I can not program the original 1MB ROM. I can
> burn
> smaller ones.
> I have zillions of 2716s and plan to use those. My ROM space requirements
> are low, and a few 2716s will do it.
> To do that, I build a wirewrap aux. board that plugs into the backplane.
> It
> will take four 2716s, and the original ROM is removed.
> Problem is, that the LS245 data chip ( U11 ) does not work. It will not
> read
> the external databus. I made a special plug that straps all the data
> lines,
> (effectively bypassing the data chip), and when I use that, everything
> works
> fine and the computer powers up with the new 2716s.
> That indicates to me that the board that I built must be without errors.
> Anyone has any ideas why the data chip will not work? I have the sneaky
> suspicion that it is something in the data-direction circuit, but I can
> not
> figure out what. All the chips check out OK, and the board powers up
> normally with the original ROM.
> BTW I checked the data chip by swapping it with an working address chip,
> it
> is OK.
> Any suggestions are appreciated.
> Bernie.
> 
>  On Feb 6, 3:12 pm, BZ80 <berniel...@wmconnect.com> wrote:
> > Hi all!
> 
> 
>